In order to prevent entry of steam at time of e.g. repair or replacement of a steam trap, an opening/closing valve needs to be disposed on upstream side (entrance side) of the steam trap. In monitoring the steam trap, it is necessary to monitor not only whether the steam trap is functioning appropriately or not, but also a functional state of the opening/closing valve. e.g. for checking whether steam is leaking to the steam trap when the opening/closing valve is closed for stopping entry of steam to the steam trap. And, conventionally, for the sake of monitoring respective functional states of the steam trap and the opening/closing valve, sensors would be attached to the steam trap and for the opening/closing valve, respectively, and based on detection information of the respective sensors, the functional states of the steam trap and the opening/closing valve would be analyzed separately (see Patent Document 1 respecting the steam trap and see Patent Document 2 respecting the opening/closing valve).